As one of key parts of engines, connecting rods during their operating processes mainly bear tension, compression, bending and other high frequency alternating loads resulted from combustion explosive force and reciprocating inertial force. Due to poor working conditions and complicated motion states, the service life of the connecting rods is affected by fatigue, abrasion, vibration and the like. Therefore, the connecting rods must have high enough strength, rigidity and comprehensive mechanical properties.
Conventional connecting rods are machined separately. After a blank of a connecting rod is forged integrally, bonding surfaces of a connecting rod body and a connecting rod cap are machined by sawing, milling, grinding and other methods. After a positioning pin hole of the connecting rod cap and a bolt hole of the connecting rod body are machined finely, the connecting rod cap and the connecting rod body are assembled together. The conventional methods have the disadvantages of redundant working procedures, low efficiency, high reject rate, and low bearing capacity and quality stability. At present, the connecting rod fracture splitting technology is the most promising machining technology in the manufacturing field of connecting rods. However, this fracture splitting technology has high requirements on the properties of material for connecting rods. The toughness of the material for connecting rods is restricted under premise of maintaining its strength, and the fractures are required to show a brittle fracture characteristic. Common material for engineering-purpose fracture splitting connecting rods is limited to powder forged material, high-carbon steel, nodular cast iron and malleable cast iron. The rigorous preparation conditions and high cost of such material limit the application and promotion of the connecting rod fracture splitting technology.
Patent No. 200820040497.X, entitled “Fracture Splitting Connecting Rod Blanks for Engines”, proposed that the powder forged material, nodular cast iron, 70 high-carbon steel or C70S6 high-carbon steel is used as material for a connecting rod, a blank of the connecting rod is molded by forging, and the connecting rod is fractured by means of fracture splitting after a stress groove is pre-machined on the inside of a large end hole of the connecting rod. Such a connecting rod has strict requirements on the strength and toughness of material, and the rigorous material preparation conditions and high cost thereof limit the application and promotion of the fracture splitting technology.
Patent No. 200580013038.1, entitled “Connecting Rods and Manufacturing Method thereof”, proposed that a segmented region of a large end portion of a connecting rod is irradiated by laser or plasma and then cooled in vacuum, so that material for the segmented region becomes martensite from austenite; and the martensite suffers brittle fracture due to fracture loads, as a result, the connecting rod body is separated from the connecting rod cap. The biggest problem of this process is that the region irradiated by laser or plasma cannot be controlled effectively. In this case, parts in the vicinity of the fracture region, in addition to the fracture region, are also embrittled. Consequently, the local mechanical strength of the connecting rod is decreased, and the fracture splitting of the connecting rod at a predetermined segmented part cannot be ensured effectively so that the fracture surfaces are likely to have offsets or other problems. This process is thus not suitable for other materials.
Patent No. 200710300307.3, entitled “Cryogenic Embrittlement Fracture Splitting Process for High-strength Alloy Steel Connecting Rods”, proposed that a connecting rod is put into liquid nitrogen for copiously cooling for more than 5 min to change the ductility of the material, so that the connecting rod becomes brittle and the brittle fracture of the connecting rod is thus realized. In this process, embrittling the whole connecting rod makes the connecting rod in a risk of decreased mechanical strength; and as the brittleness of the connecting rod increases, a large fracture load is required to segment the large end portion of the connecting rod. This makes a fracture apparatus itself large in scale and increases the equipment investments. This process is thus not suitable for other materials.
American No. US20020148325A1, entitled “Semi-solid Formed, Low Elongation Aluminum Alloy Connecting Rod”, proposed that a blank of a connecting rod is manufactured by a semi-solid forming technology, and then the toughness of the aluminum alloy connecting rod is adjusted by quenching heat treatment and artificial aging to make the aluminum alloy connecting rod meet technical requirements of the fracture splitting machining. Such a connecting rod has the following disadvantages: the preparation process of the semi-solid aluminum alloy raw material is complicated and high in cost, too large or too small crystalline grains formed after cooling will affect the toughness of the connecting rod, and the subsequent heat treatment has complex influence on the performance of the connecting rod. Consequently, it is likely to have problems during fracture splitting the connecting rod, such as torn large end, fracture splitting insufficiency, dregs and deformed fracture surfaces.
To overcome the problems and deficiencies in the above manufacturing processes of fracture splitting connecting rods, the present invention provides a method for manufacturing a composite double-metal fracture splitting connecting rod. By the composite technology, two metals of different physical, chemical and mechanical properties are metallurgically bonded at an interface to form an integral composite casting. This method compensates the deficiencies of each component material, integrates the advantages of each kind of material and realizes the diversity of the overall performance. As the connecting rod body is made of high-strength, high-toughness and high-quality material and the fracture splitting region at the large end portion of the connecting rod is made of fracture splitting material, the connecting rod is allowed to have high strength and anti-fatigue performance whilst maintaining sufficient rigidity and toughness and meets the requirements of fracture splitting division due to an external load. With the naturally staggered structures on the two fracture surfaces, repositioning and accurately assembling the separated connecting rod body and the connecting rod cap are ensured. Furthermore, the connecting rod, as without requiring mechanically machining any fitting surfaces, has high bearing capacity and high assembly accuracy.